The invention relates to an improved process for making pigment dispersions that match a standard dispersion, using the spectral transmittance properties of the dispersions in the wet to rapidly analyze the dispersions as they are being made and bring them within an acceptable match to the standard.
Single pigment dispersions or mill bases are widely used nowadays in formulating pigmented finishes, such as exterior paints for automobiles and trucks. Such dispersions are typically prepared in a milling or grinding process where the solid pigment particles are ground in the presence of solvent and polymer dispersant until a stable particle dispersion with the desired degree of fineness is formed.
It is important to carefully control these pigment dispersions with regard to tinting strength and color through particle size adjustments as they are being made, so that when they are used in specified proportions to produce a desired paint, the load color of the paint is easily shadeable/adjustable to an acceptable match to the standard color for the paint.
One difficulty, however, is that commonly used indicators of particle size, such as gloss, Hegman Gage or conventional particle size measurement, are either indirect, inaccurate or time consuming. In addition, there are no convenient methods to test tinting strength and color of neat dispersions during the milling or grinding process. Thus, acceptability of the grind is nowadays determined through traditional strength testing, which is a manual process that involves blending the dispersion with a standard white or black paint, spraying the blend onto panels, baking the panels and then comparing the panels to those of a standard batch of that dispersion blended with the same standard white or black using a spectrophotometer or calorimeter. Lightness differences between the dry sprayouts are then used as an indication of strength and acceptability of the grind.
Traditional strength testing is cumbersome and very time-consuming and the accuracy of the test is also dependent on the color and strength stability of the standard white or black paints. Even with careful control, these standards can vary from batch to batch and tend to flocculate or settle in time, leading to poor test repeatability and difficulty in accurately matching the dispersion being made to a standard dispersion. Additionally, there is inherent variability in the dry sample preparation process which also leads to inaccuracy in color determination.
Tinting strength of a dispersion is a function of both pigment particle size distribution and pigment concentration. While the traditional test indicates strength, it is unable to separate the effects of pigment concentration from particle size distribution. Thus it is impossible to determine when optimum pigment particle size has been obtained, which information could beneficially serve to minimize the amount of pigment to be used, since pigment is typically the most expensive ingredient in the paint. Lastly, the traditional test gives no indication of color changes arising from batch-to-batch pigment variability. Even when strength may equal that of the standard, the dispersion may be calorimetrically unacceptable for use in paints where it is the prime dispersion.
Therefore, there is a need for a faster and more accurate process for analyzing pigment dispersions as they are being made, that can fully characterize the properties of the dispersion and determine the end-point of the grinding process, without requiring blending and spraying with white or black standards and the production of dry samples during the manufacturing process.